Turbofan gas turbine engines for powering aircraft generally comprise inter alia a core engine, which drives a fan. The fan comprises a number of radially extending fan blades mounted on a fan rotor which is enclosed by a generally cylindrical fan casing.
To satisfy regulatory requirements, such engines are required to demonstrate that if part or all of a fan blade were to become detached from the remainder of the fan, that the detached parts are suitably captured within the engine containment system.
It is known to provide the fan casing with a fan track liner which together incorporate a containment system, designed to contain any released blades or associated debris. FIG. 1 shows a partial cross-section of such a casing and fan track liner.
In the event of a “fan blade off” (FBO) event, the detached fan blade 8 travels radially outward and forwards. In doing so, it penetrates the attrition liner 10. It may also penetrate the septum 12 and aluminium honeycomb layer 14 before engaging the hook 18. The fan track liner must therefore be relatively weak in order that any released blade or fragment thereof can pass through it essentially unimpeded and subsequently be trapped by the fan casing.
In addition to providing a blade containment system, the fan track liner includes an annular layer of abradable material which surrounds the fan blades. During operation of the engine, the fan blades rotate freely within the fan track liner. At their maximum extension of movement and/or creep, or during an extreme event, the blades may cut a path into this abradable layer creating a seal against the fan casing and minimising air leakage around the blade tips.
The fan track liner must also be resistant to ice impact loads. A rearward portion of the fan track liner is conventionally provided with an annular ice impact panel. This is typically a glass-reinforced plastic (GRP) moulding which may also be wrapped with GRP to increase its impact strength. Ice which forms on the fan blades is acted on by both centrifugal and airflow forces, which respectively cause it to move outwards and rearwards before being shed from the blades.
The geometry of a conventional fan blade is such that the ice is shed from the trailing edge of the blade, strikes the ice impact panel and is deflected without damaging the panel.
Swept fan blades are increasingly used in turbofan engines as they offer significant advantages in efficiency over conventional fan blades. Swept fan blades have a greater chord length at their central portion than conventional fan blades. This greater chordal length means that ice that forms on a swept fan blade follows the same rearward and outward path as on a conventional fan blade but may reach the radially outer tip of the blade before it reaches the trailing edge. It will therefore be shed from the blade tip and may strike the fan track liner forward of the ice impact panel within the blade off zone.
The liner used with a swept fan blade is therefore required to be strong enough to resist ice impact whilst allowing a detached fan blade to penetrate and be contained therewithin.
In recent years there has been a trend towards the use of lighter fan blades, which are typically either of hollow metal or of composite construction. These lighter blades have a similar impact energy per unit area as an ice sheet, which makes it more difficult to devise a casing arrangement that will resist the passage of ice and yet not interfere with the trajectory of a released fan blade.
It is an objective of this invention to provide a gas turbine engine containment assembly that will substantially overcome the problems described above, and that is particularly suited for use with composite, hollow metallic, or other lightweight, fan blades.